Pneumocystis pneumonia PCP) causes significant morbidity and mortality among HIV-infected individuals who are newly diagnosed, failed retroviral therapy, or do not have access to care. Infants are particularly susceptible to pulmonary infections and it has been recently appreciated that not only are individuals exposed to Pneumocystis early in life, but infants appear to carry the infection as evidenced by the high incidence found in autopsy specimens from babies that died of SIDS. HIV-infected infants tend to have a more fulminant course of PCP, likely because of the immature immune system along with depletion of CD4+ cells. In the previous funding period we found that, in contrast to our hypothesis, the delay in clearance of Pneumocystis we observed in neonatal mice was not solely due to elevated anti-inflammatory cytokines in the lungs through the first 3 weeks of life. Instead, we found that alveolar macrophages from neonatal mice are intrinsically unresponsive to Pneumocystis. The goal for this funding period is to differentiate the contribution of environmental factors and intrinsic factors to neonatal alveolar macrophage function in response to Pneumocystis. Specifically, we will test the hypothesis that: neonatal alveolar macrophages fail to respond to Pneumocystis due to a defect in signaling through pattern recognition receptors along with delayed second signals from proinflammatory cytokines or costimulatory molecules on T cells. To address this hypothesis we have proposed two aims. 1. We will determine whether alveolar macrophages from neonatal mice are intrinsically unresponsive to Pneumocystis. In vitro and in vivo experimental strategies will be used to determine whether neonatal alveolar macrophages can respond to stimuli through the dectin-1 ?-glucan receptor, mannose receptor, or TLR2. 2. We will determine whether secondary signals from T cells are required to stimulate neonatal alveolar macrophages. We will use adoptive transfer of cells and treatment with exogenous stimulatory molecules in an attempt to stimulate alveolar macrophages to respond to Pneumocystis. These studies are a logical extension of our previous funding period and will address the mechanisms responsible for neonatal unresponsiveness Pneumocystis.